In maternofetal transfer of passive immunity, IgG is transferred across cells of fetal membranous tissues. Inasmuch as the first step in the process, the binding of IgG to receptors on the cell membrane appears to be the factor which determines the overall specificity an investigation will be made into the nature of the cell membrane receptors. It will be in two parts: the preparation of soluble IgG receptor substance, and a study of its biochemical properties and binding characteristics. Solubilization of receptor substance from rabbit yolk sac membrane will be done with brief exposure of yolk sac membrane or apical brush border membrane vesicles to the action of papain. Release of IgG receptor substance from the glycocalyx will be monitored by I125-IgG binding. Gel filtration, molecular weight, association with enzyme components of the glycocalyx and the question of the importance of sialic acid for IgG binding will be among the properties studied. Measurements for IgG will be made of the binding constant, KA of the soluble receptor substance.